1. Field of the Invention
The present invention relates to an anti-reflection film which is used in order to prevent external light from reflecting on a surface of a window or a display etc. Specifically, the present invention relates to an anti-reflection film which is arranged on a surface of a display device such as a liquid crystal display (LCD), CRT display, organic electroluminescence display (ELD), plasma display (PDP), surface-conduction electron-emitter display (SED) and field emission display (FED) etc. In particular, the present invention relates to an anti-reflection film which is arranged on a surface of a transmissive LCD.
2. Description of the Related Art
In general, displays are used under an environment of incidence of external light. The external light incident to a display surface is regularly reflected and causes a decrease in display quality because the reflected image overlaps and disturbs the display's image. Thus, it is necessary to provide such a display surface with an anti-reflection function. Moreover, improving performance of the anti-reflection function and combining other functions with the anti-reflection function are also desired for such a display surface.
In general, an anti-reflection function is obtained by forming on a transparent substrate an anti-reflection layer which has a multilayer structure with repeating high and low refractive index layers made of transparent materials such as metal oxides etc. It is possible to form such a multilayer structured anti-reflection layer by a dry coating method such as a chemical vapor deposition (CVD) method and physical vapor deposition (PVD) method etc. Although the dry coating method as a forming method of the anti-reflection layer provides capability of precisely controlling each thickness of low refractive index layers and high refractive index layers, the dry coating method has a problem of unsuitability for mass production because of low productivity of the coating process under a vacuum condition. Accordingly, production of an anti-reflection film employing, as a forming method of the anti-reflection layer, a wet coating method in which a coating liquid is used so that a large-area-product production, continuous production and low-cost production are achieved is attracting attention.
In addition, because the anti-reflection film having such an anti-reflection layer on a transparent substrate has a relatively-soft surface, the anti-reflection layer is generally formed with a hard coat layer which is obtained by coating and curing an acrylic material so as to increase the surface hardness. This hard coat layer has a high surface hardness, transparency, lustrous properties and abrasion resistance as characteristics of an acrylic material.
However, the hard coat layer which is obtained by curing an acrylic material has a high level of insulation properties and is liable to be charged resulting in an occurrence of a problem of dirtiness due to dust attachment to a surface of the anti-reflection film in which the hard coat layer is formed. In order to solve such a problem, an anti-reflection film is required to have antistatic properties. In addition, in the case where an anti-reflection film having a hard coat layer and an anti-reflection layer is arranged on a surface of an LCD, an anti-reflection film with antistatic properties are also required to prevent charges on a surface of the LCD from adversely affecting inner operations of the LCD.
Thus, a method in which a conductive agent is added to the hard coat layer, and/or a method in which an antistatic layer is arranged between the substrate and the hard coat layer or between the hard coat layer and the anti-reflection layer are investigated.    <Patent document 1>: JP-A-2005-202389.    <Patent document 2>: JP-A-2005-199707.    <Patent document 3>: JP-A-H11-092750.    <Patent document 4>: JP-A-2004-004149.    <Patent document 5>: JP-A-2005-173216.    <Patent document 6>: JP-A-2005-297271.    <Patent document 7>: JP-A-2006-154758.
An anti-reflection film in which a conductive material is added to the hard coat layer to provide an antistatic function saves more production costs than that in which an antistatic layer is newly formed. In the case where a conductive material is added to the hard coat layer, however, optical properties of the resultant anti-reflection film varies according to the added conductive material.
In general, it is possible to reduce reflection of external light and improve contrast in a bright place by arranging an anti-reflection film having a hard coat layer and an anti-reflection layer on a surface of a display device to take advantage of its anti-reflection function. Then at the same time, it is also possible to brightly display an image because visible light transmittance is improved. In addition, an energy saving effect such as reduction of backlight power consumption can also be expected.
An anti-reflection film in which a conductive material is added to the hard coat layer, however, has a problem of insufficient contrast due to a decrease in luminance when displaying a white image (this type of luminance may be hereinafter referred to as “white luminance”) because an addition of the conductive material causes low visible light transmittance of the anti-reflection film.
In addition, a transmissive LCD has a problem of low contrast in a dark place caused by an insufficiently-reduced luminance when displaying a black image (“luminance when displaying a black image” may be hereinafter referred to as “black luminance”) due to a phenomenon of so-called light leakage because it is difficult to reduce orthogonal transmittance of a polarizing plate to zero. The transmissive LCD in which an anti-reflection film is arranged on the surface is provided with an anti-reflection function on the surface so that the visible light transmittance of the LCD is improved and prevention of external light reflection is achieved. The anti-reflection function and improvement of the visible light transmittance of the LCD, however, bring about an increase of light leakage when displaying a black image and particularly causes a problem of a fall in contrast in the dark place due to a high black luminance.